Process for making a multi layer polyolefin shrink film

ABSTRACT

The multi-layer polyolefin shrink film of this invention provides shrink tensions, optical clarity, cuttability, sealability, shrink temperature range, and tear resistance heretofore unobtainable in an oriented, monolayer polyolefin material. The preferred film has three layers in which the core layer comprises a blend of ethylene-vinyl acetate copolymer with ethylene-butylene copolymer and each skin layer comprises ethylene-propylene copolymer.

This is a division, of application Ser. No. 896,963, filed Apr. 17,1978.

FIELD OF THE INVENTION

This invention relates to heat shrinkable, thermoplastic packagingfilms; and in particular, this invention relates to a multi-layer,polyolefin shrink film.

BACKGROUND OF THE INVENTION

The polyolefins and polyvinyl chlorides can be considered to be the twomajor families of plastic resins from which the bulk of commerciallyavailable shrink films for wrapping purposes are made. Other resinfamilies from which shrink films can be made include the ionomers,polyesters, polystyrenes, and polyvinylidene chlorides. The shrinkablepolyolefins currently on the market are mainly monolayer films whichinclude both cross-linked and uncross-linked oriented polyethylene,oriented polypropylene, and oriented ethylene-propylene copolymers. Thepolyvinyl chloride (hereinafter "PVC") shrink films are monolayer filmsconsisting of a variety of formulations of polyvinyl chloride.

A shrink film's distinguishing characteristic is its ability uponexposure to some level of heat to shrink or, if restrained, to createshrink tension within the film. This ability is activated by thepackager when the wrapped product is passed through a hot air or hotwater shrink tunnel. This process causes the film to shrink around theproduct producing a tight, transparent wrapping that conforms to thecontour of the product and which is aesthetically pleasing whileproviding the useful functions required of packaging materials such asprotection of the product from loss of components, pilferage, or damagedue to handling and shipment. Typical items wrapped in PVC or polyolefinshrink films are toys, games, sporting goods, stationery, greetingcards, hardware and household products, office supplies and forms,foods, phonograph records, and industrial parts.

The manufacture of shrink films requires sophisticated equipmentincluding extrusion lines with "racking" capability, irradiation unitswhen cross-linking is desired, tenter frames, mechanical centerfolders,and slitters. "Racking" or "tenter framing" are orientation processeswhich cause the material to be stretched in the cross or transversedirection and in the longitudinal or machine direction. The films areusually heated to their orientation temperature range which varies withthe different polymers but is usually above room temperature and belowthe polymer's melting temperature. After being stretched, the film israpidly cooled to quench it thus freezing the molecules of the film intheir oriented state. Upon heating, the orientation stresses are relaxedand the film will begin to shrink back to its original, unorienteddimension.

The PVC and polyolefin families of shrink films provide a wide range ofphysical and performance characteristics such as shrink force (theamount of force that a film exerts per unit area of its cross-sectionduring shrinkage), the degree of free shrink (the reduction in surfacearea a material undergoes when unrestrained), tensile strength (thehighest force that can be applied to a unit area of film before itbegins to tear apart), sealability, shrink temperature curve (therelationship of shrink to temperature), tear initiation and resistance(the force at which a film will begin to tear and continue to tear),optics (gloss, haze and transparency of material), and dimensionalstability (the ability of the film to retain its original dimensionsunder all types of storage conditions). Film characteristics play animportant role in the selection of a particular film and may differ foreach type of packaging application and for each packager. Considerationmust be given to the product's size, weight, shape, rigidity, number ofproduct components, other packaging materials which may be used alongwith the film and the type of packaging equipment available.

Polyolefins have been most successful with applications where moderateto high shrink tensions are preferred; and, on new, automatic, highspeed shrink wrapping equipment where shrink and sealing temperatureranges are more clearly controlled. The polyolefins tend to be cleanerleaving fewer deposits and less residue thereby extending the life ofthe equipment and reducing equipment maintenance. The PVC filmsgenerally have better optics, lower shrink tensions, and will seal andshrink over much broader temperature ranges than the polyolefins.Polyolefins usually do not emit gases upon sealing and, in this respect,are also cleaner than PVC films.

Heretofore, polyolefins have not been able to penetrate PVC filmpackaging applications where the products to be packaged require thelower shrink tensions of the PVC film because the products are toofragile for use with polyolefins which possess shrink tensions up tofour times those of the PVC films. PVC film is also the shrink film ofchoice for older, manually operated sealers and semi-automatic wrapperswhere temperatures are highly variable. Older, poorly maintainedwrapping equipment of any type usually runs PVC better than presentmonolayer polyolefins due to the combination of the generally broadershrink and sealing temperature ranges of the PVC films. In addition,products with sharp or pointed extensions will often require PVC due tothe high initial tear resistance of the PVC film relative to that of thepolyolefins, i.e. it takes about 7 grams of force to propagate a tear inPVC whereas only 2 to 3.5 grams of force are necessary to propagate atear in a typical monolayer polyolefin shrink film.

Accordingly, it is a general object of the present invention to providea shrink polyolefin film that will have many of the desirable qualitiesof PVC films and does overcome many of PVC's limitations.

Specifically, it is an object of the present invention to provide apolyolefin film having shrink tensions approximating those of PVC filmsand also providing good optical qualities, a wide shrink temperaturerange, sealability, and resistance to tear propagation.

In addition, it is an object of the present invention to provide apolyolefin film which has none of the undesirable qualities of PVC filmssuch as noxious odors and corrosive by-products.

Furthermore, it is an object of this invention to produce a multi-layerfilm having very thin layers of oriented propylene homopolymers orcopolymers.

These and other objects are achieved by the multi-layer polyolefinshrink film which is disclosed herein.

SUMMARY OF THE INVENTION

It has been surprisingly discovered that a flexible thermoplasticpackaging film having a combination of shrink tension, optical clarity,cuttability, sealability, shrink temperature range, and tear resistanceheretofore unobtainable in a monolayer polyolefin film is achieved bythe multi-layer, flexible, thermoplastic, packaging film of the presentinvention. This multi-layer film has a "core" layer that comprises apolymeric blend, the major constituent of the blend being either ahomopolymer or copolymer of ethylene and a minor constituent beingeither a homopolymer or copolymer of butylene; and, an "auxiliary" or"skin" layer, each auxiliary layer comprising either a homopolymer orcopolymer of propylene. Preferably, the multi-layer film is oriented sothat it is heat shrinkable in at least one direction. It is alsopreferred that the auxiliary layers be formed from the same or similarpolymer and be of approximately the same thickness.

The multi-layer film may be combined with other polymeric layers forspecific applications. For instance, relatively thin layers may be addedon either or both sides of the basic three layer structure to improveseal strength or to lower gas and moisture premeability.

Preferred polymers for the major constituent are low densitypolyethylene and ethylene vinyl acetate copolymer in proportions of 70%to 90% by weight of the total blend, and preferred polymers for theminor constituent are polybutylene and ethylene-butylene copolymer inproportion of 30% to 10% by weight of the total blend.

In another aspect, the present invention is a process for making amulti-layer, flexible, thermoplastic packaging film comprising the stepsof blending a major amount of polymer selected from the group consistingof homopolymers and copolymers of ethylene and of propylene with a minoramount of a polymer selected from the group consisting of butylenehomopolymers and copolymers to form a core layer blend; providing anauxiliary layer composition comprising a polymer selected from the groupconsisting of propylene homopolymers and copolymers; coextruding saidcore layer blend between two auxiliary layers to form a multi-layerfilm; and, stretching said multi-layer film to biaxially orient same.

In yet another aspect, the present invention is a process for orientingthin films of propylene polymers and copolymers by coextruding said thinfilms with an unoriented polymeric layer therebetween.

DEFINITIONS

Unless specifically set forth and defined or limited, the term "polymer"as used herein generally includes homopolymers, copolymers, terpolymers,block, graft polymers, random, and alternating polymers.

The term "melt flow" as used herein or "melt flow index" is the amount,in grams, of a thermoplastic resin which can be forced through a givenorifice under a specified pressure and temperature within 10 minutes asdescribed in ASTM D 1238.

The term "oriented" or "orientation" as used herein means the alignmentof the molecules of a polymer predominately in a particlar direction."Orientation" is used interchangeably with "heat shrinkability" hereinand designates a material which has been stretched and set at itsstretched dimensions and which will tend to return to its originaldimensions when heated to a specific temperature below its meltingtemperature range.

The term "core" or "core layer" as used herein means a layer in amulti-layer film which is enclosed on both sides by additional orauxiliary layers. The core may be either "hot blown" or "oriented."

The term "hot blown" as used herein means that the material referred tohas been stretched at or above its melting temperature range so that thestretching has induced a minimum of stresses and molecular orientation.Such a material is not considered to be "heat shrinkable" as it willhave very low shrink tension.

"Auxiliary" layers herein refer to layers in a multi-layer film whichaid in or contribute to the shrink properties of the film.

"Skin" layers are outer layers and may also be "auxiliary" layers.

The term "polybutylene" refers to high molecular weight isotacticpolymers synthesized from butene-1 monomer. They are flexible,crystalline thermoplastic polyolefins having a density of about 0.91.They are to be distinguished from polymers of isobutene (normally termedpolybutene) which are widely used as oil additives, and from amorphousatatic poly (1-butene) polymers, which range from viscous oils torubbery polymers. "Polybutylene" is composed of linear chain moleculeshaving a regular and spatially ordered arrangement of ethyl side groupsalong the chain backbone. It adopts to helical conformation in thestable crystalline form and is normally 50 to 55% crystalline.Polybutylene is termed the "newest member of the polyolefin family" andcan be made into film, pipe and molded articles. (See "Modern PlasticsEncyclopedia 1977-1978," page 48.)

PRIOR ART STATEMENT

Closely related patents from the prior art are listed and discussedbriefly in the paragraphs which follow.

(1) U.S. Pat. No. 3,097,150 issued on July 9, 1963 to William C. Raineret al and discloses a cross-linked copolymer prepared by irradiating amixture of normally solid polyethylene and a hydro-carbon polymercontaining a plurality of isobutylene units.

(2) U.S. Pat. No. 3,176,051 issued on March 30, 1965 to Razmic S.Gregorian et al discloses a blended polyethylene composition formed byblending a major portion of a parent polymer of polyethylene having adensity of 0.94 to 0.97 and a melt index in the range of 1.0 to 10 witha minor amount between 0.1 to 10% by weight of an additive consistingessentially of a member of the group consisting of polyethylene having areduced viscosity of at least 2.9 and a copolymer of 1-butene andethylene having a reduced viscosity of 4.0 with the result that thepolyethylene composition has greatly improved clarity.

(3) U.S. Pat. No. 3,176,052 issued on March 30, 1965 to Warner L.Peticolas discloses blends of polyethylene and ethylene copolymers whichare fabricated into film. The blend comprises 5 to 75% by weight ofpolyethylene having a density of 0.91 to 0.94 blended with 95% to 25%with a copolymer of ethylene and butene-1 which has a melt index of 0.1to 10 and a density above 0.92.

(4) U.S. Pat. No. 3,381,717 issued on May 7, 1968 to Fredrick S. Tyrreland discloses a blown polypropylene tubular film wherein the core orcenter layer comprises an ethylene vinyl acetate copolymer and the outerlayers constitute a block copolymer of propylene and butylene. U.S. Pat.No. 3,595,735 which issued on July 27, 1971 also to Fredrick S. Tyrreldiscloses a similar multi-layer structure but the outer layersconstitute linear polyethylene.

(5) U.S. Pat. No. 3,694,524 issued on Sept. 26, 1972 to Harold G. Tingeret al and discloses blends of low density polyethylene andbutene/ethylene copolymers and high tear strength blown films madetherefrom. The blends have 5 to 15 weight percent low densitypolyethylene and 85 to 95% of a butene-1/ethylene copolymer whichcontains 0.25 to 3 mole percent ethylene.

(6) U.S. Pat. No. 3,817,821 which issued on June 18, 1974 to J. B.Gallini shows a three layer laminar, sealable, packaging film whereinthe first layer is a blend of ethylene vinyl acetate copolymer with asecond ethylene vinyl acetate copolymer or polybutene-1; the next orcore layer consists of high density polyethylene; and the third layer isa blend of high density polyethylene and ethylene vinyl acetatecopolymers.

(7) U.S. Pat. No. 3,821,182 issued on June 28, 1974 to William G. Baird,Jr. and discloses a method of extruding a three ply material from a diewherein saran is the center layer and the outer layers are polyethylene.This three ply material may be irradiated, biaxially oriented, and thenthe outer polyethylene layers stripped away to provide a saran film witha smooth surface.

(8) Japanese patent application Ser. No. 093,603, filed August 14, 1974and published on Jan. 5, 1978 in the name of Gunze, Ltd. discloses afilm made from a mixture of ethylene-propylene copolymer, 5 to 20% wt.of a copolymer consisting of isobutylene and containing n-butene; and0.5 to 10% wt. of an ethylene-alpha-olefin copolymer.

(9) In tables entitled "Properties of specialty films" and "Shrink andstretch film properties" on pages 37 and 39 of Modern PackagingEncyclopedia, December 1977 a coextruded ethylene-vinylacetate/polypropylene laminate and a coextrudedpolyethylene/polypropylene/low density polyethylene/polypropylene/lowdensity polyethylene laminate is disclosed as having been stretched butas having no heat shrinking characteristics. Shrink characteristics ofmonolayer polyolefins such as polyethylene, polypropylene, andethylene-vinyl acetate copolymer are listed as well as the properties ofpolyvinyl chloride.

(10) On page 7 of European Plastics News for December 1977 an articleentitled "Witco Polybutylene for Shell" states that Witco Chemical isthe only world producer of polybutylene resins and that Shell Chemicalis buying Witco's polybutylene plant which plant will use butene-1 asits feedstock.

DESCRIPTION OF THE DRAWINGS

Attached hereto and made a part of this disclosure are the drawings inwhich:

FIG. 1 is a graph of package appearance versus shrink tunnel temperaturefor packages wrapped in various shrink films;

FIG. 2 is a graph of shrink tension versus temperature for fourshrinkable wrapping materials; and,

FIG. 3 is a schematic representation of a cross section of themulti-layer film according to the present invention.

PREFERRED EMBODIMENT (A) Product

Referring to FIG. 3, the preferred embodiment of the subject inventionis a three layer, coextruded polyolefin packaging film 1 having a corelayer 2 and skin or auxiliary layers 3. The preferred core layercomprises a blend of approximately 80% by weight of ethylenevinylacetate copolymer (hereinafter designated "EVA") having approximately12% by weight vinyl acetate content and a melt flow of about 0.25 withabout 20% by weight of an ethylene-butylene copolymer having up to 5% byweight of ethylene and a melt flow of about 2.0. The preferred blendproportion range is 70% to 90% wt. EVA with 30% to 10% wt. ofpolybutylene or ethylene-butylene copolymer. The vinyl acetate contentin the EVA may vary from 8% to 20% wt. with a preferred melt flow ofless than 1.0 although melt flows up to 5.0 are usable within the scopeof the invention. Melt flow is considered a more important parameterthan vinyl acetate content as it is thought to be a significantcontributing factor in handling and machinability of the finalmulti-layer film. The melt flow of the ethylene-butylene copolymer orpolybutylene preferably ranges from about 1.0 to 3.0.

The skin layers 3 are preferably of identical composition, eachcomprising ethylene-propylene copolymer wherein the ethylene content is3.5 to 4.0% by weight and the melt flow is in the range from about 1.0to 4.0. Polypropylene may also be used as the skin or auxiliary layermaterial.

The thickness of the core layer comprises 50 to 80% of the thickness ofthe total multi-layer structure and the combined thickness of the twoskin layers comprises 20 to 50% of the total structure. The preferredtotal thickness of the multi-layer film is in the range of 0.5 to 1.5mils (0.0127 mm to 0.0381 mm) with the layer thickness ratios preferablybeing 1/2/1, 1/3/1/, 1/4/1, etc. up to 1/8/1. A "balanced" film isproduced where the auxiliary layers have the same composition andthickness, but thickness and composition may be varied within the scopeof this invention.

Looking now at FIGS. 1 and 2, the unexpected attributes of themulti-layer polyolefin shrink film of the present invention becomereadily apparent. In FIG. 1, for instance, curves of package appearanceversus tunnel temperature are plotted. The package appearance is asubjective evaluation on a scale of 1-5 and is evaluated by the samepackaging expert for all the curves in FIG. 1. The factors which enterinto the evaluation are smoothness of the shrunken film and absence ofwrinkles, gloss and haze, seal integrity, and tendency to "dog ear" atcorners and other areas of the package where shrinkage is not uniform.The tunnel temperature is the temperature of the hot air within theshrink tunnel that the wrapped packages pass through after having beenwrapped and sealed. The curves are designated "PVC" for polyvinylchloride monolayer film, specifically "Reynolon 4155" sold by ReynoldsMetals Company; "ML" for the multi-layer film of the present inventionwith a 1/4/1 thickness ratio, "PE" for monolayer, cross-linkedpolyethylene, specifically "D-Film®" sold by the Cryovac Division of W.R. Grace & Co., and "PP" for monolayer oriented polypropylene which, inthis instance, is actually an ethylene-propylene copolymer which ispredominately polypropylene sold as "CP-900" by the Cryovac Division ofW. R. Grace & Co. In this test, a gift box was packaged using an "L"type sealer. At 25° F. (14° C.) temperature intervals from 225° F. (95°C.) to 375° F. (190° C.) wrapped packages were shrunk in a hot airtunnel, an initial low temperature run at 215° F. (102° C.) having beenmade. (The curves are extrapolated from 215° F. to 200° F. in FIG. 1.)On the grading scale for the shrunken packages, 5 was consideredexcellent and 3.0 and above was considered commercially acceptable. Ascan be seen from FIG. 1 each of the films has a shrink temperature atwhich it makes an excellent package. The PVC film has a range of about110° F. (61° C.) during which its appearance is rated 3.0 or better.Likewise, the multi-layer film of the present invention has anacceptable range of from about 250° F. (120° C.) to about 340° F. (172°C.), approximately 90° F. (50° C.), during which its appearance is rated3.0 or better. On the other hand, the polyethylene shrink film has asharp peak in its appearance quality and its range above an appearanceof 3.0 is roughly from 275° F. (134° C.) to 325° F. (162° C.) which is aband of only 50° F. (280° C.). Likewise, the polypropylene film has anappearance evaluation exceeding 3.0 from roughly 280° F. (137° C.) to350° F. (176° C.) or about 70° F. (40° C.). Significantly, themulti-layer film of the present invention has a broader range ofacceptable package appearance, and its acceptable range extends to lowertemperatures than the monolayer polyolefin films.

FIG. 2 is a graph of average shrink tension in pounds per square inchversus the temperature in degrees Fahrenheit. Shrink tension isdetermined according to ASTM D 2833. The very high shrink tension ofpolypropylene is clearly shown as is also the relatively high shrinktension of the cross-linked polyethylene film. Of significance is theclose approximation of the shrink tension of the multi-layer film of thepresent invention and that of PVC. The moderate shrink tension of thefilm of the present invention is very desirable in a number of packagingapplications so that the packaged product is not distorted under theforce of the shrinking film.

The data for the multi-layer shrink film of the present invention shownin the graphs of FIGS. 1 and 2 were taken from the preferred embodimenthaving a core layer of 80% ethylene vinyl acetate and 20%ethylene-butylene copolymer. The EVA had 12% vinyl acetate and is"Alathon 3135" brand sold by du Pont having a melt flow of about 0.25.The ethylene-butylene copolymer is "Witron 8240-2" brand sold by WitcoChemical Corporation having about 2% ethylene by weight and a melt indexof about 1.0. While the 80/20 blend ratio for the core layer gives themost desirable properties, acceptable properties are generally found incore blends where the major portion of the blend is ethylenevinylacetate copolymer and the minor portion of the blend is ethylenebutylenecopolymer or polybutylene. As stated above, the range where the mostdesirable properties are found is from 70% to 90% by weight ethylenevinyl acetate with 30% to 10% by weight ethylene-butylene copolymer orpolybutylene.

The skin or auxiliary layer material for the "ML" film in FIGS. 1 and 2was ethylene-propylene copolymer "W07-1" sold by ARCO Polymers, Inc.having a melt flow index of approximately 4.0 to 5.0 and 3.5% to 4.0%ethylene content.

Additional layers or coatings can be added to the basic three layerstructure of the present invention as desired but care must be taken notto alter the desirable shrink tensions and shrink properties of themulti-layer film of this invention.

(B) Process

In the preferred process for making the multi-layer, polyolefin shrinkfilm of the present invention the basic steps are blending the polymersfor the layers, coextruding the layers to form a multilayer film, andthen stretching the film to biaxially orient it. These steps andadditional desirable steps will be explained in detail in the paragraphswhich follow.

The process begins by blending the raw materials or polymeric resins inthe proportions desired, namely for the core layer a major portion ofethylene-vinyl acetate copolymer is blended with a minor portion ofethylene-butylene copolymer. The resin is usually purchased from asupplier in pelletized form and can be blended in any one of a number ofcommercially available blenders as are well known in the art. In theblending process any additives necessary for special properties may beadded such as plasticizers, slip agents, anti-block agents, oranti-static compound.

The blended resins are fed into the hoppers of extruders which feedcoextrusion dies. For the three layer film, three extruders are employedto feed the coextrusion die. Two extruders are fed ethylenepropylenecopolymer for the two outer skin or auxiliary layers and the otherextruder is fed the blend of ethylene-vinyl acetate copolymer withethylene-butylene copolymer. Preferably the materials are coextruded asconcentric tubing having a diameter which is dependent on the rackingratio and desired final diameter. This coextruded tube is relativelythick and is referred to as the "tape." Circular coextrusion dies arewell known in the art and can be purchased from a number ofmanufacturers. In addition to tubular coextrusion, slot dies could beused to coextrude the material in sheet form; or, single or multi-layerextrusion coating could be employed.

An alternate step is to irradiate the tape or unexpanded tubing or sheetby bombarding it with high energy electrons from an accelerator tocross-link the materials in the tape. Cross-linking roughly quadruplesthe tensile strength of the film or the force at which the material canbe stretched before tearing apart when the film materials arepredominately ethylene such as polyethylene or ethylenevinyl acetate.Irradiation also improves the optical properties of the film and changesthe properties of the film at higher temperatures. The preferredirradiation dosage level is in the range of 0.5 MR to 12.0 MR. In someinstances it may be desirable to stretch the multi-layer film first thenirradiate it; or, if sequential coating is employed one layer or a groupof layers could be irradiated and then another layer or layers could beadded before the final step of stretching and orienting.

Following coextrusion and irradiation, if desired, the extruded tape isheated and is continuously inflated by air pressure into a bubblethereby transforming the narrow tape with thick walls into wide tubingwith thin walls of the desired film thickness. This process is sometimesreferred to as the "trapped bubble technique" of orientation or as"racking." After stretching, the bubble is then deflated and the film iswound onto semi-finished rolls called "mill rolls." The racking processorients the film, stretching it transversely and longitudinally therebyrearranging the molecules, to impart shrink capabilities to the film andto modify physical characteristics. In the present invention the rackingtemperature is above the melting temperature of the core or center layerwhich is comprised of the blend of ethylene-vinyl acetate copolymer andethylene-butylene copolymer as the oriented layers are theethylene-butylene copolymer layers which form the skin layers. Thus, inthe racking process the core layer is hot stretched or hot blown and theskin layers are biaxially oriented. It is believed that the hot blowncore layer provides a moderating or damping effect on the rather strongshrink properties of the ethylene-propylene layers. In addition, by thisprocess propylene or ethylene-propylene copolymer layers that are verythin are oriented which is a novel aspect of the present invention. Inother words, another aspect of the present invention is that it is aprocess for producing thin, oriented, propylene homopolymer or copolymerlayers by sandwiching a hot blown layer comprising a blend of EVA andbutylene polymer between the propylene polymer layers and then stretchorienting the composite. Also as an aspect of this invention is theproduct of the foregoing process which is a film having a very thinoriented polypropylene layer. The thickness of such a layer is in therange of 0.05 mil to 0.38 mil.

In an attempt to produce a thin, monolayer, polyolefin shrink film byracking or stretch orienting, a test procedure involving a "hat tester"was used. In using a "hat tester" to produce oriented, shrink film aheated sample of the film is placed over an orifice and differential airpressure is applied to stretch the film into the orifice. The variablesavailable in the procedure are the temperature of the film, the pressuredifferential, and the rate of of applying the pressure differential. Atemperature level of 216° F. (102° C.) was selected as the orientationtemperature because a low temperature was desired to preventself-welding of core layer and because the 216° F. level was as low asthe multi-layer film could be oriented.

First, a monolayer film, three mils thick, of the ethylenepropylenecopolymer was heated to 216° F. and pressure differentials were appliedat pressure differential steps of 1.0 p.s.i. between 1.4 and 8.4 p.s.i.The rate of applying the differential pressure was varied by achievingfull pressure in discrete time intervals from 0.6 sec. to 5.4 sec. inincrements of 0.6 sec. This film could not be stretched withoutrupturing it even when only 1.4 p.s.i. was applied in 1.8 sec.

The 3 mil ethylene propylene copolymer film sample which could not beoriented was the same material used in the skin layers of the preferredembodiment of the multi-layer film described above. Next, a 12 mil thickfilm comprising the ethylene-vinyl acetate/ethylene-butylene copolymerblend of the core layer of the preferred embodiment was subjected tostretching at the 216° F. temperature level. Again, applying pressuredifferential levels from 1.4 through 5.4 p.s.i. at 0.6 sec. incrementsfrom 0.6 sec. to 5.4 sec., the film ruptured at each attempt rather thanbeing stretched and oriented. Finally, a multi-layer sample having 3mils of the same ethylene-propylene copolymer as used above applied toeach side of the 12 mil core layer of the same ethylene-vinylacetate/ethylene-butylene copolymer blend as the preferred embodimentand was subjected to the test procedure. At the 216° F. temperaturelevel, oriented films from 0.3 mils in thickness at 7.4 p.s.i. to 1.50mils in thickness at 3.4 p.s.i. were produced using rates of applicationfrom 0.6 to 5.4 sec. at 0.6 sec. steps. Significantly, a thin, oriented,multi-layer polyolefin shrink film can be produced at a highly desirableorientation temperature at which the component layers of the multi-layerfilm can not be oriented.

Films having very thin layers of an oriented propylene polymer areproduced by the method of the present invention. For example, themulti-layer films produced are in the range of 0.5 to 1.5 mil and theauxiliary layer thickness are in the range from 0.05 mil to 0.40 mil.

EXAMPLES

In Table 1 which appears below the physical properties of sevenstructures according to the present invention are shown. In the Tablethe abbreviation "EP" is for ethylene-propylene copolymer; theabbreviation "EVA" is for ethylene vinyl acetate copolymer; "EB" is forethylene-butylene copolymer; "MD" is for machine direction; "TD" is fortransverse direction; and "M+T" is for machine+transverse. The opticaltests of haze, gloss, and transmission were performed according to ASTMD1003, the tear proprogation according to ASTM D1938, and tearresistance according to ASTM D1004.

                                      TABLE 1                                     __________________________________________________________________________      Example No.:                                                                              (1)      (2)      (3)       (4)       (5)                       __________________________________________________________________________                                    80% EVA.sup.3                                                                           80% EVA.sup.3                                                                           80% EVA.sup.3                           EP.sup.1 /EVA.sup.2 /EP.sup.1                                                          EP.sup.1 /EVA.sup.2 /EP.sup.1                                                          EP.sup.1 /20% EB.sup.4 /EP.sup.1                                                        EP.sup.5 /20%                                                                 EB.sup.4 /EP.sup.5                                                                      EP.sup.6 /20%                                                                 EB.sup.14 /EP.sup.6       __________________________________________________________________________      Layer Ratio 1/3.5/1  1/4/1    1/3/1     1/4/1     1/4/1                       Tensile X                                                                      100 (PSI)                                                                     MD         90       103      105       100       81                           TD         30       98       103       106       83                          Elongation (%)                                                                 MD         82.9     82.2     97.4      68        85.6                         TD         67.4     95.4     53.6      75.8      84.8                        Modulus X                                                                      1000 (PSI)                                                                    MD         88       93       88        89        78                           TD         99       89       110       88        95                          Tear Propaga-                                                                 ion (gms)                                                                      MD         3.79     4.21     242.0 max 24.25     18.25                        TD         3.62     5.01     12.62     25.38     19.13                     10.                                                                             Tear Resistance                                                               lbs)                                                                           MD         0.46     0.72     0.49      0.50      0.61                         TD         0.59     0.62     0.40      0.61      0.65                        Ball Burst                                                                    impact (cm-kg)                                                                            6.4      9.6      6.2       8.6       7.0                         Optics                                                                         Haze (%)   1.8      1.8      1.6       .5        2.0                          Gloss (%)  87       87       88        92        85                          Total trans-                                                                  mission (%) 92.4     92.3     92.4      92.5      92.5                      13                                                                              Shrink Tension                                                                Range (PSI)                                                                    ##STR1##   215-315  270-365  210-320   250-305   190-295                   __________________________________________________________________________                                  Example No.:                                                                              (6)       (7)                       __________________________________________________________________________                                              80% EVA.sup.3                                                                           80% EVA.sup.3                                                       EP.sup.6 /20%                                                                 EB.sup.4 /EP.sup.6                                                                      EP.sup.6 /20%             __________________________________________________________________________                                                        EB.sup.7                                              3.                                                                              Layer Ratio 1/4/1     1/4/1                                                   Tensile X                                                                     100 (PSI)                                                                     MD          93        90                                                      TD          90        86                                                      Elongation (%)                                                                MD          86.6      88.3                                                    TD          92.4      107.6                                                 3.                                                                              Modulus X                                                                     1000 (PSI)                                                                    MD          87        84                                                      TD          85        86                                                    9.                                                                              Tear Propaga-                                                                 tion (gms)                                                                    MD          23.62     23.00                                                   TD          28.88     30.25                                                 10.                                                                             Tear Resistance                                                               lbs)                                                                          MD          0.53      0.74                                                    TD          0.62      0.60                                                  11.                                                                             Ball Burst                                                                    impact (cm-kg)                                                                            8.8       7.6                                                   12.                                                                             Optics                                                                        Haze (%)    2.2       2.8                                                     Gloss (%)   87        86                                                    12.                                                                             Total Trans-                                                                  mission (%) 92.2      92.1                                                  13                                                                              Shrink Tension                                                                Range (PSI)                                                                    ##STR2##   205-290   210-290                   __________________________________________________________________________     1. Ethylenepropylene copolymer, "WO71" by ARCO Polymer, Inc. having 3.5%      to 4.0% ethylene and a melt flow of about 4.0.                                2. Ethylenevinyl acetate copolymer, "Alathon 3137" by du Pont having 12%      vinyl acetate and melt flow of about 0.5.                                     3. Ethylenevinyl acetate copolymer, "Alathon 3135" by du Pont having 12%      vinyl acetate and melt flow of about 0.25.                                    4. Ethylenebutylene copolymer, "Witron 82402," by Witco Chemical              Corporation having a melt flow range of 1.0 to 2.0.                           5. Ethylenepropylene copolymer, "A122" ARCO Polymers, Inc. having 3.5% to     4.0% ethylene and a melt flow of about 1.0.                                   6. Ethylenepropylene copolymer, "EL Hexene 4433" by Rexene Polyolefin Co.     Division of Dart Industries.                                                  7. Ethylenebutylene copolymer, "Witron 82400" by Witco Chemical               Corporation having a melt flow range of about 0.61.                           8. ASTM D882                                                                  9. ASTM D1938                                                                 10. ASTM D1004                                                                11. ASTM D3420                                                                12. ASTM D1003                                                                13. ASTM D2838                                                                14. Same as 4 except designated "Witron 82404" having melt flow of about      3.7.                                                                     

In Table II below further tests were conducted on multi-layer films ofthe present invention and PVC films to evaluate their performance underindustrial packaging conditions. (Examples designated 1, 2, 3, and 4 inTable I are the same as Examples 8, 9, 12, and 13 respectively in TableII.) In the tests rolls of each film structure were prepared andsections of the film from each roll removed and evaluated for gloss,clarity, and amount of haze and tear resistance before shrink. Each rollwas then placed on a model "W-2" packaging machine by the ShanklinCorporation and used to package a gift box as described hereinabove.

The packaged gift boxes were sent through a model "7141C" hot air shrinktunnel by Weldotron to shrink the film tightly against the boxes.Packages were run through the tunnel at 25° F. steps from 250° F. to350° F. Film was removed manually from the packages to evaluate the easeof opening and resistance to tear. In Table II, the optical propertiesare a subjective composite of all the packages made with the film ofthat particular example. For most of the films the appearance of thefilm was of such high quality that the testing methods available couldnot make objective distinctions between the films.

As is to be expected, the tear resistance of all the materials improvedafter the material was shrunk and, in general, the structures (Ex.11-18) with the ethylene-butylene copolymer blended into the core layertended to have higher levels of tear resistance as compared to theunblended core structures (Ex. 7-10).

The trim sealability of the structures in Table II was evaluated bypreparing packages and trim sealing them with an automatic "L-bar"sealer by Shanklin Corporation. The trim sealability was generallyexcellent for all coextruded structures with no noticeable buildup ofmaterial on the sealing wire and no sticking of the material to seals.Some "angel hair" did exist but it was not excessive. "Angel hair" is avery fine, thin thread-like portion of material that separates from thesealed area of the film in a trim-sealing operation. Excessive "angelhair" tends to clog the sealer, slow down the sealing operation, anddetract from package appearance. In addition, it was found that filmswith an unblended EVA core layer required longer seal dwell times thanthe corresponding examples with a blended core layer. The strength ofthe seals on the Shanklin machine were consistently in the 3 to 5lbs./linear inch range.

All the materials had acceptable packages in the shrink tunneltemperature range of 240° F. to 340° F. At temperatures above 340° F.the multi-ply materials tended to become hazy while the PVC did not.Also, the PVC films tended to have voids in the seal area while most ofthe multi-layer films according to the present invention did not.

                                      TABLE II                                    __________________________________________________________________________                                                     Trim Sealability             Example                 Optical Tear             Voids in                     No   Construction                                                                             Ratio                                                                             Haze                                                                              Gloss                                                                             Clarity                                                                           Resistance                                                                            Trim Sealability                                                                       Seal Area                    __________________________________________________________________________    8    [P.sup.1 /EVA.sup.2 /EP.sup.1                                                            1/3.5/1                                                                           None                                                                              Exc.                                                                              Exc.                                                                              Moderate                                                                              Excellent                                                                              +                            9    "          1/4/1.sup.3                                                                       None                                                                              Exc.                                                                              Exc.                                                                              Moderate                                                                              Adequate +                            10   "          1/4/1                                                                             Slight                                                                            Fair                                                                              Good                                                                              Poor    Excellent                                                                              +                            11   "          1/2/1                                                                             None                                                                              Exc.                                                                              Exc.                                                                              Poor    Excellent                                                                              +                            12   80% EVA.sup.5                                                                            1/3/1                                                                             Mod.                                                                              Poor                                                                              Poor                                                                              Poor    Excellent                                                                              +                                 EP.sup.1 /20% EB.sup.4 /EP.sup.1                                         13   "          1/4/1.sup.6                                                                       None                                                                              Exc.                                                                              Good                                                                              Moderate                                                                              Adequate +                            14   "          1/4/1.sup.7                                                                       None                                                                              Exc.                                                                              Exc.                                                                              Moderate                                                                              Excellent                                                                              None                         15   "          1/4/1.sup.8                                                                       None                                                                              Exc.                                                                              Good                                                                              Moderate                                                                              Excellent                                                                              None                         16   "          1/4/1.sup.9                                                                       None                                                                              Exc.                                                                              Good                                                                              Moderate                                                                              --       None                         17   80% EVA.sup.5                                                                            1/4/1                                                                             None                                                                              Exc.                                                                              Exc.                                                                              Poor-Moderate                                                                         Excellent                                                                              None                              EP.sup.10 /20% EB.sup.11 /EP.sup.10                                      18   "          1/4/1.sup.12                                                                      None                                                                              Exc.                                                                              Exc.                                                                              Poor-Moderate                                                                         Excellent                                                                              None                         19   "          1/4/1                                                                             None                                                                              Exc.                                                                              Exc.                                                                              Poor-Moderate                                                                         Excellent                                                                              None                         20   PVC.sup.14 1/4/1                                                                             None                                                                              Exc.                                                                              Exc.                                                                              Moderate-Good                                                                         Excellent                                                                              +                            __________________________________________________________________________     .sup.1 Ethylene-Propylene Copolymer, "WO71" by ARCO Polymers, Inc. having     3.5% to 4.0% ethylene and melt flow of about 4.0.                             .sup.2 Ethylene-Vinyl Acetate, "Alathon 3137" by du Pont having 12% vinyl     acetate content and melt flow of about 0.5.                                   .sup.3 Irradiated by electrons to a dosage of approximately 6 MR.             .sup.4 Ethylene-Butylene Copolymer, "Witron8240-2" by Witco Chemical Corp     having about 2.0% Ethylene by weight and a melt flow from 1.0 to 2.0.         .sup.5 Ethylene-Vinyl Acetate, "Alathon 3135" by du Pont having 12% vinyl     acetate content and melt flow of about 0.25.                                  .sup.6 Slip agents added to ethylenepropylene layer.                          .sup.7 Ethylene-Propylene Copolymer, "K122"]by ARCO Polymers, Inc. having     3.5% to 4.0% ethylene and melt flow of about 1.0.                             .sup.8 1.5 times slip agent concentration of (6).                             .sup.9 3.5 times slip agent concentration of (6).                             .sup.10 Ethylene-propylene Copolymer, "EL Rexene 44J3" by Rexene              Polyolefin Co. Division of Dart Industries.                                   .sup.11 Ethylene-butylene copolymer, "Witron 82404" by Witco Chemical         Corp. having melt flow of about 3.7.                                          .sup.12 Ethylene-Butylene Copolymer, "Witron8240-2" by Witco Chemical         Corp. having about 2.0% ethylene by weight and a melt flow of from 1.0 to     2.0.                                                                          .sup.13 Ethylene-Butylene Copolymer, "Witron 82400" by Witco Chemical         Corp. having melt flow index of about 0.61.                                   .sup.14 "Reynolon 4155" by Reynolds Metals Corporation.                  

EXAMPLE 21

A multi-layer film having a core layer as in Example 12 above wasprepared with skin or auxiliary layers comprising "Dypro 7649-93," anethylene-propylene copolymer from ARCO Polymes, Inc. having a melt flowof 2.3 and an ethylene content in the range of 3.5% to 4.0% by weight.This film was cross-linked by irradiating it with electrons to a dosagelevel of about 2.0 MR and then oriented by stretching it in both thelongitudinal (machine) and transverse directions. This material had hazeof 1.2%, gloss of 86%, and transmission of 92.4% as determined by themethods used in Table I. Because of the cross-linking the film wassomewhat more difficult to seal but had higher tear resistance than thefilm of Example 12. In irradiating the multi-layer film of the presentinvention, it is believed that the major portion of the cross-linkingoccurs between ethylene molecules and that the core layer would,therefore, be most affected by irradiation.

In Table III below examples 22 to 25 are shown in which the blend ratioof the core material is varied. The EVA copolymer is the "Alathon 3135"brand having 12% vinyl acetate, the EB copolymer is the "Witron 8240-2"band and the EP copolymer is the "Dypro 7649-93" brand, each of thesebrands being described hereinabove.

                  TABLE III                                                       ______________________________________                                        Example No.                                                                            22        23        24      25                                       ______________________________________                                        Core Layer                                                                             70% EVA   80% EVA   85% EVA 90% EVA                                           30% EB    20% EB    15% EB  10% EB                                   Skin Layer                                                                             EP        EP        EP      EP                                       Layer Ratio                                                                            1/4/1     1/4/1     1/4/1   1/4/1                                    Tear                                                                          Propagation                                                                   Resistance                                                                    (gms.)                                                                        MD       33.25     50.38     13.25   19.50                                    TD       46.75     60.38     27.75   24.88                                    Tear                                                                          Resistance                                                                    (lbs.)                                                                        MD       0.54      0.54      0.90    0.36                                     TD       0.66      0.48      0.82    0.51                                     Optics                                                                        Haze (%) 1.2       1.3       1.2     1.2                                      Gloss (%)                                                                              93        91        92      99                                       Shrink                                                                        Tension                                                                       Range (PSI)                                                                   MD+ TD/2 220-275   225-325   240-305 205-285                                  ______________________________________                                    

The films having a 80/20 core blend have the best combination of opticalproperties, machinability, tear propogation resistance and sealabilitywhile the films having the 70/30 core blend have lower seal strength,good to excellent optics and good tear propagation resistance. The tearpropagation resistance of the films having 85/15 core blend and of thefilms having the 90/10 core blend was not as good but these films haveexcellent seal strength and optics.

For some applications it is desirable to blend one or both components ofthe core composition into the auxiliary or skin layer composition topromote even greater melt joining of the layers, and, to some extent,further moderate the characteristics of the predominately propyleneauxiliary layer. The desirable range for this blend is 0.1 to about 15%by weight of one or more of the core constituents in the propylenehomopolymer or copolymer of the auxiliary layer.

In some instances it will be desirable to substitute polyethylene forthe ethylene vinyl acetate copolymer of the core blend and use a higherproportion of polybutylene or ethylene-butylene copolymer. In otherinstances it will be desirable to use a higher vinyl acetate content EVAwith a higher melt flow, e.g., greater than 1.0, with a smallerproportion of the butylene polymer. In general, the homopolymers andcopolymers of ethylene preferred in the core blend of the presentinvention include polyethylene, ethylene-vinyl acetate copolymer, andethylene-propylene copolymer while the homopolymers and copolymers ofbutylene include polybutylene and ethylene-butylene copolymer. Thehomopolymers and copolymers of propylene for the auxiliary layer includepolypropylene and ethylene-propylene copolymer.

An additional feature or aspect of my invention is the inclusion ofadditional polymeric layers so that a structure of 5, 7, 9, or morelayers is the result. Layers of polymers such as ionomers, polyesters,and polycarbonates may be used to impart improved hot sealcharacteristics to the basic three layer film laminate. A material issaid to have adequate "hot seal strength" if it has sufficient cohesiveand adhesive strength at heat sealing temperatures to prevent bondfailure under the stress applied to the seal while it is still hot.Also, layers of polymers such as vinyl chloride-vinylidene chloridecopolymer (saran) and hydrolyzed ethylene-vinyl acetate copolymer may beused if low gas permeability is desired.

EXAMPLE 26

Based on work done and experience with the multi-layer films a fivelayer film can be prepared with the following preferred thickness ratiofor the layers: 1/2/1/2/1. The outer or skin layers are theethylene-propylene copolymer of the preferred embodiment above. Thecentral layer is "Suryln" brand ionomer sold by duPont. Ionomers are, ofcourse, polymers having ethylene as a major component but with bothcovalent and ionic bonds. The remaining two layers are the preferredcore composition according to the present invention. In other words,this multi-layer film structure is the same as the preferred three layerstructures described above except that the central or core layer hasbeen divided in half and a layer of ionomer interposed therebetween.This structure will retain the desirable shrinkage properties of thepreferred three layer laminate and will, in addition, have added "hotseal strength" due to the ionomer layer.

EXAMPLE 27

The same structure as described in Example 26 is used except that thecentral layer is a polyester, preferably polyethylene terephthalate,rather than an ionomer. Again, the desirable shrink properties of thethree layer film are retained plus the "hot seal strength" is improved.

EXAMPLE 28

The same structure as described in Example 26 is used except that thecentral layer is formed from a polycarbonate resin. The shrinkproperties of the preferred embodiment plus the "hot seal strength" areavailable in this laminate.

Other multi-layer structures can be made within the scope of myinvention by including special purpose polymeric layers with the basiccore and auxiliary layers described above.

Having thus described my invention,
 1. A process for making amulti-layer, flexible, thermoplastic packaging film comprising the stepsof:(a) blending 70% to 90% by weight of polymer selected from the groupconsisting of homopolymers and copolymers of ethylene with 30% to 10% byweight of a polymer selected from the group consisting of butylenehomopolymers and copolymers to form a core blend; (b) providing anauxiliary composition comprising a polymer selected from the groupconsisting of propylene homopolymers and copolymers; (c) coextruding alayer of said core blend between two layers of said auxiliarycomposition to form a multi-layer film, said core layer being 1 to 8times the thickness of said auxiliary layers; and, (d) stretching saidmulti-layer film to orient same.
 2. The process of claim 1 wherein themajor portion of said core layer blend is ethylene-vinyl acetatecopolymer having a melt flow of less than 5.0 and said minor portion isethylene-butylene copolymer.
 3. The process of claim 1 wherein saidauxiliary layer comprises ethylene-propylene copolymer.
 4. The processof claim 1 including between steps (c) and (d) the steps of cooling saidfilm and then reheating same to its orientation temperature.
 5. Theprocess of claim 4 wherein said multi-layer film is coextruded as a tubeand said stretching and orientation is accomplished by inflating saidtubing by the trapped bubble technique to biaxially orient same.